This invention relates to power plants whose primary energy source is a nuclear reactor. The invention has particular relationship to plants including steam generators in which steam is generated by heat interchange between the reactor coolant flowing through primary tubes and water in the outer secondary shell enclosing the tubes. The coolant is at a high pressure typically 2000 pounds per square inch. The water in the shell and the steam which is generated is at a substantially lower pressure. Typically the coolant is circulated through a large number of Ushaped tubes in heat-interchange relationship with the water in the shell.
In the operation of such nuclear power plants, it sometimes happens that one or more of the tubes is ruptured. The rupture may be a hole in a tube or a tube may be severed completely. The complete severing of a tube is described as a "double-ended rupture". A double-ended rupture is referred to as a design-basis tube rupture because it is one of the factors which must be considered in the design of a nuclear power plant. In the case of any rupture, whether a double-ended rupture or a hole in a tube coolant which is radioactive is injected, under the pressure in the coolant system, into the water in the outer secondary shell.
A double-ended rupture results in a critical emergency because the steam-generator shell is rapidly filled with water both because of emergency feedwater flow and flow of coolant from both ends of the ruptured tube into the shell. On the occurrence of a hole in a tube, as distinct from a double-ended rupture, the emergency is less critical; the steam generator shell does not fill as rapidly as for a double-ended rupture. However, coolant is in this case injected into the liquid in the shell under the high pressure in the perforated tube and action must at some time be taken to preclude overflow. The established design considerations postulate the double-ended rupture of more than one tube. The severity of the emergency is necessarily increased on the occurrence of a multiple double-ended rupture.
The radioactivity of the fluid emerging from the steam generator shell is monitored. On manifestation of a substantial increase in radioactivity the operator of the plant is appraised of a possible rupture. Responsive to an increase in radioactivity the operator checks the rise in level of the secondary liquid in the steam generators to determine if there is a rapid rise in any generator indicating which generator has failed. The auxiliary feedwater flow is readily terminated to any generator which shows a rapid rise but the flow into the secondary shell through any ruptured tube or tubes presents difficulties. In accordance with the teachings of the prior art, the coolant or primary side of the steam generator is, on the occurrence of a rupture, cooled down by the discharge of steam from the valves on the shell side or secondary of the steam generator and by the tripping of the reactor. In addition the coolant is depressurized so that its pressure is below the pressure of the fluid on the secondary side. While the operator is waiting for these time consuming processes to be culminated, coolant is pouring into the shell side of the steam generator. Coolant which flows out of the rupture is replenished in the core by the safety-injection system, which is also enabled, on the occurrence of a rupture, to preclude overheating of the core so that the injection of coolant into the shell side of the steam generator continues without interruption.
A double-ended rupture is classified as a condition IV design-basis event by the Nuclear Regulatory Commission. Current NRC guidelines for such an event recommend that no operator action be required during the first 30 minutes after the occurrence of a condition IV event in the interest of giving the operating personnel time to overcome the shock and possible panic of the occurrence of the event and to evaluate what has happened and what action to take. But analysis reveals that under prior-art practice, the operator must begin to take action no later than 10 minutes after the occurrence of a doubleended rupture. Delay would result in steam generator overflow and flooding of the steam lines supplied by the generator. Steamline flooding not only menaces the structural integrity of the steamlines but the resulting water flow through the secondary safety facilities and poweroperated relief valves may prevent these valves from reseating and engender release of radioactivity into the environment exceeding the limits set out in 10 Code of Federal Regulatories 100.
It is an object of this invention to overcome the drawbacks and disadvantages of the prior art and to provide a method for effectively precluding overflow of a steam generator of a nuclear reactor and flooding of the steamlines supplied by the generator on the occurrence of a rupture of a tube or tubes which conduct the coolant. It is also an object of this invention to provide a nuclear reactor power plant in whose operation this method shall be practiced on occurrence of a tube rupture.